FIG. 3 shows a specific structure of a conventional linear encoder. FIG. 4 is a cross sectional view along line B-B′ in FIG. 3. As shown in the cross section in FIG. 4, a scale unit 1 of the conventional linear encoder shown in FIG. 3 includes a scale enclosure 2 that is open at and around a corner defined by the bottom surface thereof and the rear surface thereof, and has a main scale 3 secured inside the scale enclosure 2. The main scale 3 is made of glass, and has a scale made of metallic thin film and carved in matrix on the surface thereof at a constant pitch in the longitudinal direction. A slider unit 20 of the linear encoder includes a slider enclosure 21 roughly including a slider holding unit 22, a detection head holding unit 23, and a pillar 24 that connects the two. The detection head holding unit 23 has a detection head secured thereon including a light-emitting unit 7, a mirror 8, and a light-receiving unit 29. In the detection head, the light emitted from the light-emitting unit 7 is reflected on the mirror 8 at the right angle to be irradiated into the matrix portion of the main scale 3, and the transmitted light is converted into an electric signal by the light-receiving unit 29. The slider holding unit 22 is secured on a moving unit or the like of a machine, using a bolt. The slider holding unit 22 has a built-in circuit board 27 for converting the electric signal from the light-receiving unit 29 into a position data signal. A through hole 26 is formed in the pillar 24 and the detection head holding unit 23 so that an electric wire 28 passes through the through hole 26 from the detection head to the circuit board 27. The position data signal outputted by the circuit board 27 is outputted to the outside via a waterproof connecter 9 mounted on the slider holding unit 22. A cover 4 is securely attached to the slider holding unit 22 to protect the circuit board 27 from water, oil, or the like.
The scale unit 1 and the slider unit 20 are assembled to each other, as shown in FIG. 3, and the detection head, the detection head holding unit 23, and a part of the pillar 24 are accommodated in the scale enclosure 2. Seals 5, 6 are fixedly attached near the opening of the scale enclosure 2. The tip ends of the seals 5, 6 are in contact with each other to thereby close the opening of the scale enclosure 2 to prevent intrusion of dust, water, oil, or the like from the outside. The pillar 24 is long in the longitudinal direction of the scale unit 1, and thin in the width direction, and has a cross section having a ship-like shape. With this shape, the pillar 24 moves while breasting the two seals 5, 6 in the advancing direction as the slider unit 20 moves. Further, with this shape, the tip ends of the seals 5, 6 breasted are brought into contact with each other again on the opposite side of the advancing direction of the pillar 24.
The slider enclosure 21 including the slider holding unit 22, the detection head holding unit 23, and the pillar 24 is integrally molded using metal, such as aluminum or the like, generally by means of lost wax casting, die casting, or the like. However, it is not possible to form the through hole 26 of the detection head holding unit 23 and the pillar 24 by means of integral molding, as the diameter of the hole is very small while the length thereof is as long as five times the hole diameter. Further, in the case where the through hole is formed in post processing, there are available only drill machining and discharge machining with high machining cost, and it is necessary to form a plurality of holes when there are many wires. Still further, as the pillar is very thin, machining defect may likely be caused at the time of drill machining by a drill by breaking through the surface of the pillar. Yet further, while a task of passing a plurality of electric wires through the through hole of the pillar, a task of soldering for connecting the electric wire passing through the through hole to the electric circuit, and a task of pressing the connecter terminal for attachment are necessary, the number of steps required for wiring also presents a problem. Note that as a method for improving a wiring task, there is available a method that uses an FCC (a flexible flat cable) instead of an electric wire. However, in order to pass an FCC through the through hole, it is necessary to form a long hole in the pillar, and formation of such a hole in post processing requires repetitive execution of discharge machining and drill machining. Further, there is available a method, as a method for forming a long hole in a pillar, that forms a part of the pillar of the slider enclosure, using two molded components having a shape divided by a long hole. This method, however, has a problem of reduced strength of the pillar that supports the detection head holding unit.
The present invention has been conceived in view of the above, and an object of the present invention is to implement a slider structure of a linear encoder in which a through hole of a detection head holding unit and a pillar of a slider enclosure is formed by means of integral molding or milling machining, to provide a linear encoder with a lower cost.